The bleed structure may be used in stationary gas turbine engines, but is especially advantageous for aircraft jet engines. Jet engine is meant to include various types of engines, which admit air at relatively low velocity, heat it by combustion and shoot it out at a much higher velocity. Accommodated within the term jet engine are, for example, turbojet engines and turbo-fan engines. The invention will below be described for a turbo-fan engine, but may of course also be used for other engine types.
An aircraft gas turbine engine of the turbofan type generally comprises a forward fan and booster compressor, a middle core engine, and an aft low pressure power turbine. The core engine comprises a high pressure compressor, a combustor and a high pressure turbine in a serial relationship. The high pressure compressor and high pressure turbine of the core engine are interconnected by a high pressure shaft. The high-pressure compressor, turbine and shaft essentially form a high pressure rotor. The high-pressure compressor is rotatably driven to compress air entering the core engine to a relatively high pressure. This high pressure air is then mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows aft and passes through the high-pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the high pressure compressor.
The gas stream leaving the high pressure turbine is expanded through a second or low pressure turbine. The low pressure turbine rotatably drives the fan and booster compressor via a low pressure shaft, all of which form the low pressure rotor. The low pressure shaft extends through the high pressure rotor. Most of the thrust produced is generated by the fan.
Part of the incoming air flow to the aircraft engine enters an inner, primary gas duct, which guides the air to the combustor, and part of the incoming air flow enters an outer, secondary gas duct (fan duct) in which the engine bypass air flows.
In known aircraft engines, a bleed passage extends between the primary gas duct and the secondary gas duct. According to a known configuration, a variable bleed passage system is adapted to bleed air from the primary gas duct to the secondary gas duct. In certain operational conditions, compressed air is bled from the primary gas duct via the bleed passage and introduced in a high speed gas flow in the secondary gas duct.
There is a risk that the bleed air will negatively effect the stability or efficiency of the engine or cause vibration problems. A small air cushion is created when the bleed air meets the gas flow in the fan duct, which locally increase the pressure in the forward end of the outlet. This increased pressure creates a non-uniform distribution of the bled gas flow, which leads to losses. More specifically, for a set extension of the outlet in the axial direction of the engine, the bleed gas will only flow into the gas duct through a small part of the outlet at the downstream end of the outlet.
It is desirable to achieve a bleed structure for a gas turbine engine, which creates conditions for an effective bleed while not negatively influencing the operation of the engine or at least keep the negative effects to a minimum. More specifically, it is desirable to improve the flow distribution in the bleed passage with no substantial negative effects on the gas flow in a gas duct from which the air is bled and/or in a gas duct into which the bled air is introduced.
In accordance with an aspect of the present invention, the first and second wall portions end at different positions in an extension direction of the bleed passage opening. Thus, the first and second wall portions end at different positions in a direction of the bleed flow in the bleed passage. In other words, the first and second wall portions end at different positions in a direction perpendicular to a plane in parallel to the walls defining the opening.
Such an opening configuration at a bleed passage outlet creates conditions for a more favorable pressure distribution in a gas flow in the bleed passage. Likewise, such an opening configuration at a bleed passage inlet creates conditions for a more favorable pressure distribution in the bleed passage.
The opening configuration is especially advantageous in applications for bleed between a primary gas duct and a secondary gas duct where a pressure difference is small between a compressor portion and the secondary gas duct (fan duct) in order to secure bleed through-flow to a sufficient extent and in the intended direction. The opening configuration is further advantageous in applications where there is a limited space available for the bleed opening.
According to an aspect of the invention, for a bleed passage outlet, an upstream wall portion ends at a position closer to a wall defining the gas duct, which is opposite said bleed passage opening, than the downstream wall portion. The speed of the introduced bleed gas may then be levelled to some extent at the outlet in the axial direction of the gas turbine and a larger bleed flow may be introduced than according to prior art. In other words, the bleed gas will flow into the gas duct through a larger part of the outlet.
Thus, according to an aspect of the invention, one of the first and second wall portions is raised relative to the adjacent surfaces of the structure. This opening configuration at the outlet creates conditions for introducing a large bleed air flow into the gas duct.
According to a further aspect of the invention, the other of the first and second wall portions is flush with the adjacent surfaces of the structure. This opening configuration at the outlet creates conditions for substantially not negatively effecting the passing gas flow in the gas duct into which the bleed air is introduced.
According to a further aspect of the invention, one of the first and second wall portions is lowered relative to the adjacent surfaces of the structure. This opening configuration at the inlet creates conditions for substantially not negatively effecting the passing gas flow in the gas duct from which the bleed air is extracted.
According to a further aspect of the invention, a transition from at least one of said first and second wall portion to an adjacent gas duct wall is even so that any disturbance caused by bleed on a passing gas flow is minimized. The transition portion is preferably smooth, uninterrupted and substantially flat.
According to a further aspect of the invention, it comprises at least one airfoil in said bleed passage opening for guiding a gas flow in the passage. By virtue of the airfoils, the bleed air may he guided in a desired direction to/from the bleed passage. Further, the airfoils create conditions for a larger deflection of the bleed flow in a set axial distance.
Further advantageous embodiments and further advantages of the invention emerge from the detailed description below.